MOS-controlled power devices have become increasingly important in the design of integrated circuits. In the prior art, a MOSFET gate has been used to turn off a thyristor by short circuiting the emitter-base junction of the NPN transistor. This thyristor structure, however, exhibits poor current turn-off capability. A relatively new development is the depletion-mode thyristor (DMT) in which trenches are etched in a vertical thyristor device and filled with a conducting material to form gates. By applying voltage to the gates, a depletion region may be induced in the vertical thyristor which will turn off the device.
While the DMT has some advantages over prior art thyristor circuits, it also has several disadvantages. First, since the device is built on bulk silicon, trench etching must be used to form the gates which control the depletion region. The etching and refill processing is fairly complicated, thereby reducing yield and reliability of the devices. A further disadvantage is that a common P+ substrate used as the emitter of the PNP transistor makes it impossible to put several of the thyristors on a chip, since the P+ substrate/PNP emitter will be common to all the thyristors.
Therefore, a need has arisen in the industry to provide a thyristor which does not require trench processing and which can be fabricated such that other devices on the chip are not affected by its operation.